Typical aircraft turbofan jet engines include a fan that draws and directs a flow of air into and around an engine core and a nacelle. The nacelle surrounds the engine core and helps promote the laminar flow of air past the engine core. The flow of air that is directed into the engine core is initially passed through a compressor that increases the air flow pressure, and then through a combustor where the air is mixed with fuel and ignited. The combustion of the fuel and air mixture causes a series of turbine blades at the rear of the engine core to rotate, and in turn to provide power to the fan. The high-pressure heated exhaust gases from the combustion of the fuel and air mixture are thereafter directed through an exhaust nozzle out of the rear of the engine.
The flow of air that is directed around the engine core is called bypass flow and provides the main thrust for the aircraft. The bypass flow also is used to help slow an aircraft, when the flow is diverted by thrust reversers mounted in the nacelle structure that surrounds the engine core. The bypass flow may or may not be mixed with the engine core exhaust before exiting.
Several turbofan engine parameters are important to those of skill in the art in order to optimize design characteristics and performance. The bypass ratio (BPR) is the ratio of air mass passing through the fan to that going through the core. Higher BPR engines can be more efficient and quieter. In general, a higher BPR results in lower average exhaust velocities and less jet noise at equivalent thrust rating of a lower BPR engine. Also, the exit area and mass flow rates and pressures define the fan pressure ratio (FPR).
Turbofan engine operation parameters and characteristics can further be reflected in a turbofan engine's operating map. Operation maps can be created in various ways, such as on turbine rig test results or predicted by applicable computer programs as is known in the art. Typical turbine operation maps can show relationships between pressure ratios (e.g., FPR) on the y-axis and corrected mass flows on the x-axis. The operation line(s) on the turbofan operation map reflects the line or ranges in which the relationship between FPRs and correct mass flow values result in maximum thrust and minimum fuel consumption. For example, it is known that altering the engine's characteristics that lower the operating line can increase fuel efficiency and reduce noise emissions from the engine since more thrust is produced with less fuel being injected into the combustors, and the stoichiometry of the engine is increased. The resulting reduction of FPRs, however, can reach a practical limit as a low FPR can cause engine fan stall, blade flutter or compressor surge under certain operating conditions, with insufficient bypass flow possibly causing engine malfunction.
A solution to optimizing the operating line at all flight conditions, for those engines that draw significant benefit from such an optimization, includes varying the exit nozzle area during operation. Variable area nozzles for aircraft jet engines are known to help aircraft obtain lower FPR by favorably reconfiguring engine cycle characteristics. Such variable area nozzles generally have included a series of flow deflectors or fins (often called “turkey feathers”) that can flair outwardly or pivot inwardly to increase or decrease the size of the nozzle opening and accordingly expand or constrict the flow of the exhaust air upon exit. Unfortunately, the expansion of such turkey feathers may cause undesirable leakage and can adversely interact with the outside air flow passing over the engine, which can create undesirable drag, noise, and a reduction in thrust due to the overboard leakage, leading to greater fuel consumption. In addition, prior variable area nozzle systems typically have been heavy, expensive and somewhat complex in their structure and operation, generally requiring the coordinated movement of multiple components that employ complicated drive mechanisms.
Accordingly, it can be seen that a need exists for a variable area nozzle assembly for an aircraft turbine engine that promotes a cost effective, simple and efficient operation for control of engine output to match desired flight conditions.